Claude review: drm/i915/display: Error codes for async flip failures

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**`DRM_MODE_ATOMIC_INVALID_API_USAGE` for unsupported pixel format/modifier:**
```c
drm_mode_atomic_add_error_msg(&state->base.error_code,
    DRM_MODE_ATOMIC_INVALID_API_USAGE,
    "[PLANE:%d:%s] pixel format %p4cc / 0x%llx modifier does not support async flip",
```
Using `INVALID_API_USAGE` for "this hardware doesn't support this format/modifier combo with async flip" seems wrong. This isn't invalid API usage; the API is being used correctly but the hardware can't do it. `DRM_MODE_ATOMIC_SPEC_VIOLOATION` (typo aside) seems more appropriate, or a new code.

**`%p4cc` format specifier in `kvasprintf`:** The `%p4cc` format is a kernel-specific `printk` extension for fourcc codes. `kvasprintf` should support it since it uses the kernel's `vsnprintf`, but this is worth verifying. It will produce something like `XR24 little-endian` in the error string -- is that useful for userspace?

**Only partial coverage:** Only a few error paths in `intel_async_flip_check_uapi` and `intel_async_flip_check_hw` are converted. Many other error returns in these functions still use `drm_dbg_kms` without setting an error code. This means userspace will get `DRM_MODE_ATOMIC_UNSPECIFIED_ERROR` (0) for those cases, which defeats the purpose.

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